It’s bulletproof glass for the two small blind spots the Secret Service can’t cover. They tried different agent setups to cover all sides first, but eventually figured „ah, fuck it, let’s just put some bullet proof glass on these two angles and we’re fine.“ Also, they have a secondary use to project text for the president to read. But that only came up after Obama came up with idea. „Hey guys, why don’t you project the text I need to read somewhere in the wall, so I don’t have to use a stack of papers?“ - „but isn’t that too far away to read?“ - „ oh, yeah… hey, how about we use that bullet proof glass thingy and project it on there?“ - „seriously? Projecting onto glass? What did they put in your cereal this morning, sir?“